1. Field of the Invention
The present invention relates to a coiled optical assembly made of a long optical fiber and a fabricating method for the same; and, in particular, to a dispersion compensator for reducing the wavelength dispersion of an optical fiber transmission line in the wavelength band of 1.55 xcexcm, and a fabricating method for the same.
2. Related Background Art
Long-distance, large-capacity transmission is possible in the wavelength band of 1.55 xcexcm by means of an optical amplifier operable in the wavelength band of 1.55 xcexcm utilizing an optical fiber doped with erbium (Er) which is a rare-earth element. However, when transmission is carried out in the wavelength band of 1.55 xcexcm by using a single-mode optical fiber whose zero-dispersion wavelength is in the 1.3-xcexcm band (1.3SMF), which is employed in conventional optical transmission lines, then a large wavelength dispersion occurs and distorts optical signals, thereby deteriorating the signal quality. As a consequence, when carrying out transmission in the 1.55-xcexcm band with the use of 1.3SMF, its wavelength dispersion is required to be kept low. Known as one of techniques therefor is a method using a dispersion-compensating optical fiber (DCF) having a large wavelength dispersion with a polarity opposite to that of the dispersion of 1.3SMF, so as to cancel the wavelength dispersion in the 1.55-xcexcm band.
In a technique employed for carrying out such dispersion compensation with DCF in a conventional long-distance transmission line, a dispersion compensator having a compact size in which a long DCF is wound around a bobbin is installed at every repeater station.
Nevertheless, since a wide-band DCF has a large bending loss in general, it may yield a large transmission loss in the 1.55-xcexcm band, i.e., transmission wavelength band, when formed into a coil having a small diameter. This bending loss can be reduced when the coil has a larger diameter so that its number of turns is reduced. Increasing the coil diameter, however, is unfavorable in that the dispersion compensator accordingly becomes larger.
Also, the dispersion compensator is often used together with an optical amplifier using an erbium-doped optical fiber. In this case, the temperature of the dispersion compensator increases due to the heat from a pumping laser within the optical amplifier, so that the bobbin may thermally expand. As a result, distortions may occur in the wound wide-band DCF, thereby increasing the transmission loss. Using a material with less thermal expansion in the bobbin can reduce the transmission loss in such a high-temperature environment. However, materials having low coefficients of thermal expansion, such as silica glass, ceramics, special alloys, and the like, are hard to process or expensive.
Therefore, it is an object of the present invention to provide an optical assembly accommodating therein a long optical fiber in a compact fashion, which yields less increase in transmission loss upon such bending or heat; and a method of making the same.
In order to achieve the above-mentioned object, the inventors carried out various studies using the wide-band DCF, results of which will be discussed here.
FIG. 1 is a sectional view of a DCF constituting an optical fiber coil studied. As shown in FIG. 1, the DCF employed has an optical fiber 11 made of glass and two coating layers 13, 15, each made of a resin, formed around the optical fiber 11. FIG. 2 shows the refractive index profile of this DCF. The glass portion 11 is a double-cladding type DCF whose core portion has a diameter a of 2.65 xcexcm, depressed cladding portion has a diameter b of 7.58 xcexcm, and outside diameter c is 100 xcexcm. The primary coating layer 13 has a thickness d of 20 xcexcm. The secondary coating layer 15 has a thickness e of 20 xcexcm. The outside diameter f of the fiber is 180 xcexcm. The relative refractive index differences xcex94+, xcex94xe2x88x92 of the core portion and depressed cladding portion with respect to the refractive index of the outer cladding portion were set to 2.1% and xe2x88x920.4%, respectively. At 20xc2x0 C., the Young""s modulus of the primary coating layer 13 was 0.06 kgf/mm2, and that of the secondary coating layer 15 was 65 kgf/mm2. The wavelength dispersion and wavelength dispersion slope of this DCF were xe2x88x92100 ps/nm/km and xe2x88x920.29 ps/nm2/km at the wavelength of 1.55 xcexcm, respectively, whereas its transmission loss was 0.40 dB/km.
FIG. 3 is a perspective view of a take-up bobbin 2 used for producing the optical fiber coil. Around the bobbin 2 made of aluminum having a body portion 20 with a diameter g of 100 mm, flanges 21 with a diameter of 200 mm, and a winding width k of 18 mm, the above-mentioned DCF having a fiber length of 10 km was wound at a winding pitch of 0.4 mm with a take-up tension of 40 gf, so as to produce the optical fiber coil.
The respective transmission characteristics of thus obtained optical fiber coil (type 1) wound around the bobbin, the optical fiber coil (type 2) loosened into a bundle form after being removed from the bobbin, and the optical fiber coil (type 3) obtained after that of type 1 had been subjected to a predetermined heat treatment were measured and compared with each other.
FIG. 4 shows the heat cycle of the heat treatment applied to the optical fiber coil of type 3. In this heat treatment, the optical fiber coil of type 1 was left for 1 hour at a temperature of 20xc2x0 C., subsequently the temperature was raised at a rate of 1xc2x0 C./minute until it reached 80xc2x0 C., at which the optical fiber coil was left for 1 hour, and then the temperature was lowered at a rate of 1xc2x0 C./minute until it reached xe2x88x9240xc2x0 C., at which the optical fiber coil was left for 1 hour. After this cycle was repeated once again,the optical fiber coil was finally maintained at 20xc2x0 C. and left for 2 hours.
FIG. 5 is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types 1 and 2 with those of the optical fiber before being wound up. In the optical fiber coil of type 1, a large transmission loss (1.7 dB at 1.55 xcexcm) occurred in the 1.55-xcexcm band, i.e., transmission wavelength band, and the transmission loss became greater as the wavelength was longer. It is due to the microbend loss occurring when the optical fiber is bent with a small curvature. By contrast, this microbend loss substantially disappeared from the optical fiber coil of type 2. From these facts, the inventors have found that the transmission loss generated upon the winding of a coil is mainly caused by distortions in winding due to a multiplex winding, e.g., lateral pressures applied to each fiber from its adjacent fibers, which cause the optical fiber to bend, thereby generating a microbend loss upon coil winding. Hence, the inventors have concluded that an optical fiber coil having a low transmission loss can be produced if these lateral pressures are eliminated.
When the optical fiber coil of type 2 was heated to 70xc2x0 C. and then its transmission loss was measured at a wavelength of 1.55 xcexcm, the measured value was greater than that at 20xc2x0 C. by 0.06 dB. This small amount of change in transmission loss value is comparable with the value of 1.3SMF reported in a literature (Tanaka et al., xe2x80x98TEMPERATURE DEPENDENCE OF INTRINSIC TRANSMISSION LOSS FOR HIGH SILICA FIBERxe2x80x9d, European Conference on Optical Communication, pp. 193-196, 1987). Therefore, this transmission loss is considered to be the temperature-dependent loss inherent in the optical fiber, which is a value irrelevant to the lateral pressures.
FIG. 6 is a graph comparing, at each wavelength, the transmission loss values of the optical fiber coils of types 1 and 3 with those of the optical fiber before being wound up. In the optical fiber coil of type 3, the amount of change in loss was reduced to that of the optical fiber coil of type 1, whereby its amount of change in transmission loss at a wavelength of 1.55 xcexcm was 0.25 dB. though the amount of change in transmission loss of this optical fiber coil at a wavelength of 1.55 xcexcm further increased by 0.24 dB when heated to 70xc2x0 C., it was still lower than that of the optical fiber coil of type 1 at 20xc2x0 C., i.e., 1.7 dB.
Thus, the inventors have found that subjecting a coil to a heat treatment can reduce its lateral pressures and also lower the temperature dependence of its amount of change in transmission loss.
Therefore, the present invention is configured as follows.
The coiled optical assembly made of a long optical fiber in accordance with the present invention comprises an optical fiber coil in which the long optical fiber is formed into a coil and adjusted into a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more, and a storage case accommodating the optical fiber coil.
Since the optical fiber coil is accommodated in the storage case in a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more so as to substantially release distortions in winding, the lateral pressure applied by each layer of the optical fiber to another layer of the optical fiber due to multiplex winding of the long optical fiber is alleviated. Also, when wound around a bobbin, the optical fiber is less likely to be influenced by the thermal expansion of the bobbin even in a high-temperature environment.
When the long optical fiber has a wavelength dispersion and a wavelength dispersion slope which have polarities opposite to those of the wavelength dispersion and wavelength dispersion slope of the optical fiber constituting a transmission line, respectively, then the coiled optical assembly functions as a dispersion compensator for reducing the wavelength dispersion in a predetermined wavelength band. As a consequence, a dispersion compensator having a favorable characteristic can be obtained.
The storage case may accommodate the long optical fiber in a bundle form. When the coil-shaped optical fiber is loosened into a bundle form and accommodated in the storage case, then its distortions in winding can substantially be released in a simple manner.
Alternatively, the optical fiber coil may be wound around a bobbin. In this case, the lateral pressures applied to the optical fiber from its adjacent layers of optical fiber or the bobbin are alleviated, and the optical fiber coil is accommodated so as to be free from the influence of the thermal expansion of the bobbin even when placed in a high-temperature environment.
The bobbin may be made of a metal. In this case, by forming the optical fiber coil in a state where the bobbin is heated and expanded, and then cooling and shrinking the bobbin, the optical fiber coil can be adjusted into a state where distortions in winding are substantially released.
When the winding pitch of the optical fiber coil is set to a value at least twice as large as the diameter of the long optical fiber, then mode coupling occurs between orthogonal polarization modes, whereby polarization mode dispersion (PMD) can be reduced.
The diameter of the optical fiber coil may be 100 mm or less in its smallest portion. In such a state which is substantially free of distortions in winding, it is possible to produce an optical assembly using a small-sized optical fiber coil having a diameter of 100 mm or less, which has conventionally been difficult.
Preferably, the long optical fiber has a coating layer on the outer peripheral surface thereof. Preferably, this coating layer is the one in which a primary coating layer made of a coating material having a Young""s modulus of at least 0.03 kgf/mm2 but not greater than 0.15 kgf/mm2 and a secondary coating layer made of a coating material having a Young""s modulus of at least 50 kgf/mm2 but not greater than 100 kgf/mm2 are laminated, or a coating layer made of a coating material having a Young""s modulus of at 1 kgf/mm2 but not greater than 120 kgf/mm2. Preferably, the coating layer has a thickness of at least 20 xcexcm but not greater than 70 xcexcm.
When such a coating layer is provided, then it becomes easier to coil the long optical fiber in a state substantially released from distortions in winding.
The optical assembly of the present invention may further comprise a coil-tidying member which secures the optical fiber coil to the storage case or bobbin, thereby preventing the optical fiber coil from becoming disordered in winding.
When the coil-tidying member secures the optical fiber coil to the storage case or bobbin, then, even when the optical fiber coil is accommodated in the state substantially released from distortions in winding, it is possible to prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.
The coil-tidying member may be formed by a resin which secures the optical fiber coil to the storage case or bobbin at a plurality of positions. Alternatively, it may be a cushion material securing the optical fiber coil to the storage case. The coil-tidying member may further comprise a filler for securing and holding the optical fiber coil. As the filler, a thermosetting or UV-curable silicon gel having a Young""s modulus of 0.05 kg/mm2 or less upon curing or a jelly-like admixture having a high viscosity is preferable.
When the optical fiber coil is thus secured to the storage case or bobbin by means of the coil-tidying member, it is possible to easily prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.
Also, a cushioning filler may fill gaps between turns of the optical fiber constituting the optical fiber coil. As this filler, a thermosetting or UV-curable silicon gel having a Young""s modulus of 0.05 kg/mm2 or less upon curing or a jelly-like admixture having a high viscosity is preferable.
When the cushioning filler fills gaps between turns of the optical fiber, then the lateral pressures generated by the turns of the optical fiber acting on each other are alleviated, whereby distortions in bending are suppressed. Further, this filler can easily prevent the optical assembly from vibrating upon transportation or the like, the optical fiber from breaking upon impacts, the wound optical fiber coil from becoming disordered, and the transmission loss from thereby increasing.
On the other hand, the fabricating method for a coiled optical assembly made of a long optical fiber in accordance with the present invention comprises a coil making step of winding the long optical fiber around a bobbin so as to make an optical fiber coil; and an adjustment step of adjusting the optical fiber into a state where the amount of increase in transmission loss in a predetermined wavelength band upon this coil making step is reduced by 0.1 dB/km or more.
Thus, as the optical fiber is placed into a state where the amount of increase in transmission loss in a predetermined wavelength band upon this coil making step is reduced by 0.1 dB/km or more in the adjustment step so as to substantially release distortions in winding, the coiled optical assembly in accordance with the present invention can be produced.
For example, this adjustment step can employ any of: (1) a step of holding, after the coil making step, the optical fiber coil at at least one of temperatures which are lower and higher than the temperature in the coil making step, respectively; (2) a step of vibrating the bobbin wound with the optical fiber coil after the coil making step; and (3) a step of removing the optical fiber coil from the bobbin and loosening thus removed optical fiber coil into a bundle form.
In the step (1), which is derived from the above-mentioned finding, the coil is subjected to a heat treatment, so as to reduce lateral pressures, whereby the coil is adjusted into the state where distortions in winding are substantially released. In the step (2), the bobbin is vibrated so as to homogenize the winding state of the coil, thereby making the lateral pressures uniform and adjusting the optical fiber coil into the state substantially released from distortions in winding. In the step (3), the coil is loosened into a bundle form, so as to be adjusted in the state substantially released from distortions in winding.
The method may further comprise a filling step of accommodating the optical fiber coil in a storage case and filling the storage case with a cushioning filler. When the storage case is thus filled with the cushioning filler, the optical fiber coil can be secured and held without excess lateral pressures being applied thereto.
The method may further comprise a lubricant coating step of coating the bobbin with a lubricant prior to at least one of the coil making step and adjustment step. This is effective in that the friction between the bobbin and the optical fiber coil can be reduced, whereby it is possible to reduce lateral pressures and enhance the elimination of distortions in winding upon the adjustment step.
In another aspect, the fabricating method for a coiled optical assembly in accordance with the present invention comprises a coil making step of forming an optical fiber coil by winding a long optical fiber around a bobbin in a state where the amount of increase in transmission loss in a predetermined wavelength band upon coiling is reduced by 0.1 dB/km or more. Preferably, this coil making step comprises at least one of (1) a step of winding the long optical fiber around the bobbin in a state where at least one of the long optical fiber and the bobbin is maintained at a predetermined temperature not lower than 60xc2x0 C.; and (2) a step of winding the long optical fiber around the bobbin at a take-up tension of 50 gf or less.
The step (1) can alleviate lateral pressures which may occur after the bobbin or optical fiber is cooled. The step (2) can form a coil in a state substantially released from distortions in winding.
In still another aspect, the fabricating method for a coiled optical assembly comprises a coil making step of winding a long optical fiber around a bobbin so as to make an optical fiber coil; and a body diameter changing step of substantially reducing, after the coil making step, the diameter of the bobbin from the diameter thereof in the coil making step, so that the optical fiber is adjusted into a state where the amount of increase in transmission loss in a predetermined wavelength band upon winding is reduced by 0.1 dB/km or more.
When the body diameter is thus substantially reduced in the body diameter changing step, so as to form an optical fiber coil in a bundle form in which a wide-band DCF is loosened, then the lateral pressures generated due to a multilayer of winding are alleviated, whereby the optical fiber coil that is not influenced by thermal expansion of the bobbin even when placed in a high-temperature environment can reliably be made.
Further, the method of making an optical assembly in accordance with the present invention may comprise a filler coating step of coating an outer periphery of a long optical fiber with a cushioning filler, and a coil making step of winding the long optical fiber, whose outer periphery is coated with the filler, around a bobbin, so as to make an optical fiber coil; or a coil making step of making an optical fiber coil while coating, simultaneously with winding the long optical fiber around a bobbin, the surface of the bobbin for winding the long optical fiber and the outer periphery of the wound long optical fiber with a cushioning filler.
In this manner, since the cushioning filler is disposed between turns of the optical fiber and between the optical fiber and the bobbin, the lateral pressures applied to the optical fiber are alleviated, whereby the optical fiber coil that is not influenced by thermal expansion of the bobbin even when placed in a high-temperature environment can securely be made.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.